This invention relates generally to magnetic resonance imaging (MRI), and more particularly, the invention relates to multi-echo MRI
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials and is an approach to medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
Magnetic resonance (MR) imaging is based on nuclear spins, which can be viewed as vectors in a three-dimensional space. During an MRI experiment, each nuclear spin responds to four different effects: precession about the main magnetic field, nutation about an axis perpendicular to the main field, and both transverse and longitudinal relaxation. In steady-state MRI experiments, a combination of these effects occurs periodically.
Reliable and uniform fat suppression is clinically important, as bright fat signals may obscure underlying pathology and degrade the diagnostic value of MR images. Compared to fat suppression methods based on spectrally-selective excitation and STIR (short-tau inversion recovery), multi-point water-fat separation methods based on chemical-shift induced phase differences are less susceptible to magnetic field inhomogeneities. In order to correct field inhomogeneities before water-fat separation, the latter group of methods often incurs long scan times to acquire multiple images for field map estimation. Long scan times result in suboptimal imaging efficiency and motion-induced artifacts, which prohibit routine clinical use of multi-point water-fat separation methods for many applications.
The problems due to long scan times are partially addressed by multi-echo sequences, which acquire multiple echoes in a single repetition. Multi-echo sequences have been used for water-fat separation in applications that require highly signal-to-noise ratio (SNR) efficient imaging, such as abdominal imaging and flow-independent angiography. However, the echo-spacings in multi-echo sequences are typically much longer than those in single-echo sequences. Long echo-spacings impair robustness of field map estimation, and hence result in less reliable water-fat separation. In other words, reliable water-fat separation with multi-echo sequences requires short echo-spacings. However, short echo-spacings dictate large receiver bandwidth and/or low spatial resolution, both of which cause a loss of SNR. Therefore, the application of multi-echo sequences to water-fat separation is still limited.
In some processes a plurality of scans in k-space are used. The scans are performed in the same direction, so between scans a fly-back gradient is used to return the scan parameters to an initial position or region.